1. Technical Field
The present invention relates to a switching power source device and a control IC for lighting LEDs which are capable of performing constant power control.
2. Background Art
FIG. 1 is a configuration block diagram showing a conventional LED lighting device described in Japanese Patent Application Publication No. 2006-210836. In FIG. 1, an EMI filter 1 is connected between both terminals of an AC power source AC, and an output terminal of the EMI filter 1 is connected to an input terminal of a full-wave rectifying circuit DB configured to rectify an AC voltage of the AC power source AC. An output terminal of the full-wave rectifying circuit DB is connected to a capacitor Cin. The full-wave rectifying circuit DB and the capacitor Cin form an input-side rectifying-smoothing circuit. A stage subsequent to the capacitor Cin is formed by a flyback converter having a transformer TR, a switching element Q1, a control integrated circuit (IC) 10, and an LED-group load device 3.
The transformer TR has a primary winding W1, a secondary winding W2, and an auxiliary winding W3. The secondary winding W2 and the auxiliary winding W3 are wound to have reversed phase compared to that of the primary winding W1. The switching element Q1 is formed with a MOSFET, and is connected to the primary winding W1 of the transformer TR and driven by the control IC 10 configured to generate an on/off drive signal.
An output-side rectifying-smoothing circuit formed by a series circuit of a diode D1 and a capacitor C1 is connected between both ends of the secondary winding W2 of the transformer TR, and the LED-group load device 3 having series-connected LED1 to LEDn is connected between both terminals of the capacitor C1. A rectifying-smoothing circuit formed with a series circuit of a diode D2 and a capacitor C2 is connected between both ends of the auxiliary winding W3 of the transformer TR, and a voltage generated at the capacitor C2 is supplied as a power source voltage Vcc for the control IC 10.
To perform constant current control on a load current flowing through the LED-group load device 3, a constant current circuit (CC circuit) is formed, and a detected load current is inputted, as a feedback signal FB, to a current-control error amplifier (not shown) in the control IC 10 so that the load current may have a predetermined average current value.
However, when the constant current control is performed on currents for LEDs whose forward voltages VF largely vary, power inputted to the LEDs varies, causing variations in the amount of luminous flux among individual lighting devices.
In some cases, to suppress the variations in the amount of luminous flux, constant power control is performed on the LED loads. In the constant power control on the LED loads, a constant current IF is decreased when the forward voltage VF of an LED is high while the constant current IF is increased when the forward voltage VF is low. In this case, a computation of VF×IF=constant needs to be performed by circuitry. For example, VF×IF=constant can be achieved by using a multiplier or the like to generate an FB signal having information for constant power.
When VF×IF=constant is achieved using the multiplier or the like, a single path is formed for the FB signal by placing the multiplier or the like at the secondary side of the transformer TR, but a photo coupler or the like is needed to form an insulating power source. Thus, the circuit configuration becomes complicated. Moreover, it is difficult with a conventional circuit technology to perform the constant power control with a simple circuit other than the multiplier or the like.
An objective of the present invention is to provide a switching power source device and a control IC which are inexpensive and small and are capable of performing constant power control without a multiplier and a photo coupler and reducing variations in the amount of luminous flux even when the forward voltages VF vary.